Field of the Invention
The invention relates to a method for activating an X-ray detector. The invention also relates to a control unit for implementing such a method.
Description of the Prior Art
A basic requirement of X-ray imaging, in particular in the field of computed tomography (CT), is the signal stability and reproducibility. Ideally the X-ray detector should always supply the same measuring result in response to the same input radiation. In reality the detector response is influenced by a number of factors, however. Long-term influencing variables are ageing processes, the radiation dose of the incident X-ray radiation, and varying operating conditions as a result of on/-off switching cycles. An X-ray detector is subject to a short-term (temporary) influence due to the radiation history and as a result of changes in temperature (which may, in turn, be due to radiation). In computed tomography, variations in the detector response (with constant incident X-ray radiation) are usually called “drift”.
Detectors known as direct converting, quantum-counting X-ray detectors are increasingly being used, particularly in computed tomography. An X-ray detector of this kind conventionally has an X-ray sensitive sensor layer in which electron-hole pairs are generated by incident X-ray quanta. The radiation-induced electron-hole pairs are separated by applying a depletion voltage. For this purpose, a control electrode extending over the entire detector surface is applied to a front side, usually facing the radiation source, of the sensor layer, and this is connected to a negative electric potential. An arrangement of defined electrodes is disposed on the back of the sensor layer that faces away from the front. These electrodes on the back, of which each generates one pixel of the X-ray image to be recorded, and which are therefore called “pixel electrodes” below, are usually connected to ground.
Under the effect of the depletion voltage, typically on the order of magnitude of 1,000 volts, the “knocked out” electrons drift, due to an X-ray quantum, to one of the pixel electrodes and activate a current pulse there. The term “depletion” in this sense denotes the removal of the radiation-induced free electrons from the sensor layer. The current pulses generated at the individual pixel electrodes are detected as a counting event by an electronic evaluation device connected downstream. The electronic evaluation device usually subjects the peak current of the detected current pulses to a threshold analysis in order to distinguish real, i.e. radiation-induced, counting events from other interference currents. Sometimes the electronic control device carries out a graduated threshold comparison, during the course of which the current pulses are counted according to the current peaks in different channels. Since the current peaks (pulse level) of the radiation-induced current pulses depends on the quantum energy of the detected X-ray quanta, this enables a spectral (i.e. activated according to the frequency or, equally, the quantum energy) detection of the detected X-ray radiation.
In X-ray detectors, in particular X-ray detectors of the type described above, the drift routinely has a spatial distribution over the sensor surface. The detector response is therefore spatially inhomogeneous, with spatial homogeneity of the incident X-ray radiation. The static portion of this spatial inhomogeneity is usually compensated by taking a reference image with homogeneous irradiation and by division by the relative counting rate in conventional computed tomography. This compensation method leaves the causes of the inhomogeneity of the detector response unaffected, however. Furthermore, this method is not suitable for compensation of the short-term drift (i.e. the short-term change over time in the detector response). Instead, this may currently be reduced only by improved sensor materials or by conditioning of the sensor material (for example by means of infrared radiation). These measures are only effective to a limited extent, however, and are often associated with a significant increase in the production and operating costs.